In PEM fuel cell systems, various hydrocarbon compounds may be used as a source of fuel. Commonly used fuels include alcohols, such as methanol and ethanol, and formic acid. Formic acid generally boasts a number of advantages over alcohol-based fuels when used in a PEM fuel cell application, including low flammability, low toxicity, good ionic conductivity, ability to generate hydrogen ions under anhydrous conditions, high mass transfer to the anode, and relatively low fuel crossover through Nafion™ membranes.
Direct formic acid fuel cell (“DFAFC”) systems produce electricity through the following anodic and cathodic reactions:Anode: HCOOH→CO2+2H++2e−  (1)Cathode: ½O2+2H++2e−→H2O  (2)
Prior art DFAFC system designs include systems that employ fuel “dosing” operations that use a motorized metering pump to rapidly deliver a set dose of fuel to an anode; the fuel volume is distributed across the anode due to a high inlet fuel velocity, and the fuel is wicked and retained by a series of “fuel layers” adjacent to an anode catalyst. Fuel is slowly delivered to the anode catalyst from the fuel layers until the onset of fuel starvation is detected; when fuel is depleted from the anode chamber, an anode catalyst regeneration sequence can be initiated or the metering pump can deliver more fuel. A state of fuel depletion is required to reach the target anode potential range for regeneration. Some problems with such DFAFC systems include reduced energy production between fuel doses, which necessitates onboard energy storage for bridging power, and the addition of a power supply for the anode regeneration sequence. Also, complex electronics are required for this sequence which also must be powered by electricity produced by the fuel cell.
Because the energy density of formic acid is lower than alcohol-based fuels, it is desirable to minimize or outright eliminate parasitic power loss from support components in a DFAFC, such as motorized pumps and power conversion electronics which are powered by electricity produced by the fuel cell. DFAFCs with no electrical power consuming components are herein referred to as passive fuel cells. A number of known challenges in designing passive DFAFCs, especially those that use relatively high concentrations of formic acid, include: catalyst poisoning, formic acid crossover from the anode to the cathode through the membrane, restrictive diffusion barriers within the gas diffusion layer, and dehydration of the membrane. Passive DFAFCs also present challenges in materials selection, MEA assembly, fuel management, low electrical resistance current collection, robust integration of the array or stack, and methods for manufacture.
It is desirable to provide a passive PEM fuel cell design applicable to at least a DFAFC that addresses at least some of the challenges in the prior art.